Raman-Scattering Line Shape of the SoftEPolariton Mode in BaTiO3

We report observation of Raman scattering in the polariton region by the soft $E$ mode in tetragonal BaTi${\mathrm{O}}_3$. The system has excellent characteristics for testing the validity of theories that predict polariton line shapes as a function of wave number. Our spectral data are quite well fitted in the entire polariton region by a treatment recently reviewed by Barker and Loudon, in which the Nyquist theorem is applied to a simple phenomenological model of the crystal using a constant damping factor $\Gamma$. In addition, we point out that the Green's-function approach by Benson and Mills yields a line-shape result that here reduces to that of the Barker—Loudon model, if it is assumed that the real part of the self-energy is zero and the imaginary part is proportional to frequency.